This is a genetic-epidemiological study that will quantitate the influence of familial (genetic and environmental) factors on the development of sleep apnea. The degree to which obstructive sleep apnea aggregates within families will be determined. Furthermore, the extent to which the observed familial aggregation of sleep apnea is related to specific structural and physiologic traits common to family members will be assessed. The study population will consist of 75 subjects identified through the sleep laboratory with obstructive sleep apnea, and 75 ag and gender-matched control subjects identified through industry, and the first-degree relatives of both groups. During home visits, questionnaire data (e.g., symptoms, medical history, and exposures) will be collected, and the following measurements will be made: blood pressure, height, weight, and spirometry. Structural assessment of the upper airway will be made by a brief physical examination, and facial structure will be documented with a lateral photograph.. Airflow, chest wall movement, oxygen saturation, and heart rate during sleep will be recorded with an ambulatory monitoring device. Observations in the field will be confirmed and extended with laboratory studies on a sample of families who demonstrate the greatest and the least concordance for sleep-related respiratory disturbances. These subjects will undergo more detailed assessment of upper airway structure with cephalometry and posterior rhinometry, assessment of ventilatory control with responses to chemical and resistive loading, and assessment of sleep staging with in-hospital polysomnography. Familial correlations with and without adjustment for specific risk factors will be computed. These analyses will allow 1) determination of risk of the development of sleep apnea conferred by familial factors, 2) an improved understanding of the influences of genetic and acquired risk factors (and their interactions) on the development of sleep apnea, and 3) characterization of a generally healthy population at increased risk for sleep apnea that may be studied subsequently both longitudinally (in natural history studies) and with molecular genetic markers (in pedigree studies).